Activated carbon is used in gas adsorption applications. Once the adsorption capacity of the activated carbon is completely utilized the carbon has to be regenerated by removing or desorbing the adsorbed species.
The desorption process depends on the adsorption potential for the particular species which in turn is determined by the size of the gas molecule, its polarizability as well as the mean distance between the graphitic platelets in the activated carbon structure. In general, if platelet distance is more than 3 or 4 molecular diameters the adsorption potential is low and so adsorbed species can be desorbed easily. If the distance is less than 3 molecular diameters the adsorption potential is high and the adsorbed species cannot be desorbed easily.
The species which are not adsorbed strongly can be easily desorbed by flowing a current of air at low temperatures. For strongly adsorbed species however, the carbon has to be heated to increase the vapor pressure of the adsorbed gases and decrease the adsorption potential. A current of heated air or steam can be passed through the carbon to desorb the gases.
Typically steam or heated air regeneration has to be carried out in a separate reactor. In applications in which activated carbon has to be repeatedly regenerated, frequent steam or air regeneration is expensive and inconvenient.
Desorption has been carried out on granular carbon beds by passing an electric current through the carbon. However, there are disadvantages to passing an electric current through granulated carbon beds. Because there is no continuous contact between carbon granules, that is, there are open channels between the granules which are necessary for the flow of gases, there is no way to have uniform current flow through the granules. Since resistance varies within the same granular bed along a given flow path and also from path to path, heating can cause hot spots and desorption can occur at different rates. Resistance changes as a function of time due to the unsymmetrical attrition of the granules and therefore is not uniform. As the temperature of the carbon increases, resistance decreases and uncontrolled heating can result which can cause fires.
The magnitude of the electric current through the granular bed at a given cross section depends on the resistance offered by the granules in its path. For example, the more dense the carbon granules, the lower will be the resistance and hence the higher the current through that path for a given applied voltage. This type of situation will lead to hot spots in the bed.
There remains a need to have activated carbon adsorber in a form in which adsorbed gases can be easily, efficiently, safely, and economically desorbed.
The present invention provides such an activated carbon adsorber.